Optical communications based on the use of waveguide optical fibers have become a commercially viable alternative to transmission by means of wire pairs, coaxial cables, and microwave broadcasting at least for point-to-point connections, e.g., between major urban centers and between high-capacity computer facilities. Suitability of fibers for such connections is predicated on well-known fiber characteristics, among which are low loss, small size, low weight, high bandwidth, and near-perfect electrical isolation. These characteristics are potentially significant also in communications networks in which more elaborate interconnection is required such as, in particular, in so-called local area networks (LAN).
One relevant aspect in the design of local area networks is the need for passive connections between transmission lines, e.g., for feeding signals from a multiplicity of end stations onto a fiber serving as a "bus", as well as tapping into such fiber to extract a signal intended for a specific station or "node". Desirably, a tap is made so as to remove only a small fraction of the power traveling on a bus. For the sake of efficiency of a feed, however, it is desirable to effect transfer of all or almost all of the locally available power.
This desired coupling characteristic, namely high coupling efficiency for signal insertion, combined with tapping of but a small fraction of power, has been found impossible to realize because proposed optical couplers are subject to the principle of reciprocity, i.e., the requirement that the fraction of local transmitter power injected into the bus be equal to the fraction of power split off the bus into the local receiver. And, since the coupling fraction for tapping must be relatively small, reciprocity requires that only a corresponding small fraction of the local transmitter power be injected into the bus, thereby wasting a large percentage of transmitter power. Typical in this respect are systems as disclosed in the following papers:
C. A. Villarruel et al., "Tapped Tee Single-mode Data Distribution System", IEEE Journal of Quantum Electronics, Vol. QE-17 (1981), pp. 941-946;
T. Ozeki et al., "Optical Directional Coupler Using Tapered Sections in Multi-mode Fibers", Applied Physics Letters, Vol. 28 (1976), pp. 528-529; and
M. K. Barnoski et al., "Fabrication of an Access Coupler with Single-strand Multi-mode Fiber Waveguides", Applied Optics, Vol. 15 (1976), pp. 2629-2630.
One attempt at designing a coupler arrangement not subject to the reciprocity principle is disclosed in the paper by H. H. Witte, "Passive T-Bus with Asymmetrical 4-Port Couplers", Fiber and Integrated Optics, Vol. 3 (1980), pp. 253-261, such arrangement including optical waveguides joined end-to-end with a lateral offset of the optical axes. However, the improvement to be expected with this approach as compared with couplers subject to the reciprocity principle was found to be relatively small; moreover, resulting couplers can be expected to have high coupling loss.
Accordingly, there remains a need for a low-loss coupler which provides for feeding essentially all power from a local node into a fiber serving as a bus, while tapping but a small portion of power from the bus.